Hydrothermal synthesis can be defined as a method of synthesis of single crystals that depends on the solubility of minerals in hot water under high pressure. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called an autoclave, in which a nutrient is supplied along with water. A temperature gradient is maintained between the opposite ends of the growth chamber. At the hotter end the nutrient solute dissolves, while at the cooler end it is deposited on a seed crystal, growing the desired crystal.

Possible advantages of the hydrothermal method over other types of crystal growth include the ability to create crystalline phases which are not stable at the melting point. Also, materials which have a high vapour pressure near their melting points can also be grown by the hydrothermal method. The method is also particularly suitable for the growth of large good-quality crystals while maintaining good control over their composition. Disadvantages of the method include the need of expensive autoclaves, and the impossibility of observing the crystal as it grows.[2]

Synthetic quartz crystals produced in the autoclave shown in Western Electrics pilot hydrothermal quartz plant in 1959.

The first report of the hydrothermal growth of crystals[3] was by German geologist Karl Emil von Schafhäutl (1803-1890) in 1845: he grew microscopic quartz crystals in a pressure cooker.[4] In 1848, Robert Bunsen reported growing crystals of barium and strontium carbonate at 200 °C and at pressures of 15 atmospheres, using sealed glass tubes and aqueous ammonium chloride ("Salmiak") as a solvent.[5] In 1849 and 1851, French crystallographer Henri Hureau de Sénarmont (1808-1862) produced crystals of various minerals via hydrothermal synthesis.[6][7] Later (1905) Giorgio Spezia (1842-1911) published reports on the growth of macroscopic crystals.[8] He used solutions of sodium silicate, natural crystals as seeds and supply, and a silver-lined vessel. By heating the supply end of his vessel to 320-350 °C, and the other end to 165-180 °C, he obtained about 15 mm of new growth over a 200 day period. Unlike modern practice, the hotter part of the vessel was at the top. A shortage in the electronics industry of natural quartz crystals from Brazil during World War 2 led to postwar development of a commercial-scale hydrothermal process for culturing quartz crystals, by A. C. Walker and Ernie Buehler in 1950 at Bell Laboratories.[9] Other notable contributions have been made by Nacken (1946), Hale (1948), Brown (1951), and Kohman (1955).[10]

A large number of compounds belonging to practically all classes have been synthesized under hydrothermal conditions: elements, simple and complex oxides, tungstates, molybdates, carbonates, silicates, germanates etc. Hydrothermal synthesis is commonly used to grow synthetic quartz, gems and other single crystals with commercial value. Some of the crystals that have been efficiently grown are emeralds, rubies, quartz, alexandrite and others. The method has proved to be extremely efficient both in the search for new compounds with specific physical properties and in the systematic physicochemical investigation of intricate multicomponent systems at elevated temperatures and pressures.

The crystallization vessels used are autoclaves. These are usually thick-walled steel cylinders with a hermetic seal which must withstand high temperatures and pressures for prolonged periods of time. Furthermore, the autoclave material must be inert with respect to the solvent. The closure is the most important element of the autoclave. Many designs have been developed for seals, the most famous being the Bridgman seal. In most cases steel-corroding solutions are used in hydrothermal experiments. To prevent corrosion of the internal cavity of the autoclave, protective inserts are generally used. These may have the same shape of the autoclave and fit in the internal cavity (contact-type insert) or be a "floating" type insert which occupies only part of the autoclave interior. Inserts may be made of carbon-free iron, copper, silver, gold, platinum, titanium, glass (or quartz), or Teflon, depending on the temperature and solution used.

The most extensively used method in hydrothermal synthesis and crystal growing. The supersaturation is achieved by reducing the temperature in the crystal growth zone. The nutrient is placed in the lower part of the autoclave filled with a specific amount of solvent. The autoclave is heated in order to create two temperature zones. The nutrient dissolves in the hotter zone and the saturated aqueous solution in the lower part is transported to the upper part by convective motion of the solution. The cooler and denser solution in the upper part of the autoclave descends while the counterflow of solution ascends. The solution becomes supersaturated in the upper part as the result of the reduction in temperature and crystallization sets in.

In this technique crystallization takes place without a temperature gradient between the growth and dissolution zones. The supersaturation is achieved by a gradual reduction in temperature of the solution in the autoclave. The disadvantage of this technique is the difficulty in controlling the growth process and introducing seed crystals. For these reasons, this technique is very seldom used.

This technique is based on the difference in solubility between the phase to be grown and that serving as the starting material. The nutrient consists of compounds that are thermodynamically unstable under the growth conditions. The solubility of the metastable phase exceeds that of the stable phase, and the latter crystallize due to the dissolution of the metastable phase. This technique is usually combined with one of the other two techniques above.

^Laudise, R.A. (1958). R.H. Doremus, B.W. Roberts and D. Turnbull, ed. Growth and perfection of crystals. Proceedings of an International Conference on Crystal Growth held at Cooperstown, New York on August 27–29, 1958. Wiley, New York. pp. 458–463.